You cannot add a single One SL as a surround speaker to the Beam. It has to be a pair. But you can GROUP the One SL with the Beam.

Follow these instructions: https://support.sonos.com/s/article/3391?language=en_US

What is the difference between that and Grouping the speakers? 
 

If I were to add another ONE SL and do surround sound, what is the main difference this will provide, over, doing GROUP of the speakers

A group plays the same thing.
 

A surround pair of speakers plays the surround signal from a 5.1 mix, which plays sounds that would come from ‘behind’ you, as a watcher. 

Note that speakers ‘grouped’ with a Sonia home theater room will have a slight delay, only when using the TV. If you’re streaming music, they will be in perfect sync. If they are surround speakers, they’re part of the home theater room, and will be in sync at all times. 
 

Not everything is broadcast in 5.1, although any streaming movies should be, unless they’re from before the technology existed. There are times that surround speakers will be ‘quiet’. Not frequently, but it can happen. 

Adding a pair of One SLs as surrounds to the Beam creates a 5.0 home theater surround sound system. The two surround speakers will play the left and right rear surround channels from a 5.1 audio track.

Grouping one or more speakers to the Beam will just result in the same audio playing from all grouped speakers. If you group the One SL to the Beam when playing TV audio, you will have a very slight delay in the audio from the One SL. But playing music from the grouped Beam and One SL will be in sync.